The goal of the proposed work is to determine the incidence and predictors of musculoskeletal injuries in a cohort of free-living adult women. There are approximately 40-50 million sedentary adults in the U.S. at present, and public health and medical authorities recommend that these persons accumulate at least 30 minutes of moderate intensity physical activity per day. The most prevalent risk associated with physical activity is musculoskeletal injury, with an incidence of activity-related injury as high as 25 percent per year in community dwelling adults. Women athletes suffer higher rates of injury than their male counterparts, but there is insufficient information available on injury incidence in representative population samples of free-living individuals, especially in women. Biomechanical and orthopedic factors such as excessive Q-angle or foot pronation are plausible putative risk factors for activity-related injury in women athletes; however, empirical evidence of these associations is lacking in non-competitive physical activity settings. The public health burden of physical activity-related injuries that might ensue if all sedentary women began to exercise at recommended levels is currently unknown. The focus in the proposed prospective observational study will be on quantifying the public health burden of physical activity related injuries, and to identify demographic, orthopedic, biomechanical, body habitus, and physical activity- related risk factors for musculoskeletal injury. Strengths of the study will be the recruitment of a population of 885 women broadly representative of the community, an extensive baseline clinical assessment of possible risk predictors, ongoing surveillance of both physical activity habits and injury outcomes, confirmation of injury events by medical record review, and calculation of injury-related costs. We will collect information pertaining to sedentary, moderate, and vigorous physical activity behaviors that might be effect modifiers or confounding factors of the association between the emerging risk predictors and injury risk. Findings from this study will provide empirical evidence upon which strategies for injury prevention can be developed and current health-related activity recommendations can be refined to minimize the risk of injury among susceptible women who are physically active.